The present invention relates to vehicle braking systems, and in particular to braking systems for trailers and towed vehicles.
Although it is common for vehicles to pull light trailers without the addition of a brake system to the trailer, heavier trailers and other towed vehicles generally require a separate braking system due to their inertia. These heavier trailers and vehicles include, by way of example, house trailers, recreational trailers, horse trailers, car trailers, and the like. (All such trailers and other towed vehicles will be generally referred to henceforward simply as “trailers.”) Without supplemental braking systems for heavier trailers, it may be difficult or impossible for an operator to maintain control of a vehicle-trailer combination during braking. This problem is particularly acute with respect to recreational trailers, since they may be pulled by non-commercial vehicles with less-capable braking systems, and because recreational trailers are commonly pulled through steeply inclined mountainous areas that are popular destinations for recreational vehicle owners. In addition, various state laws contain minimum requirements concerning the braking ability and functionality of recreational trailers and other trailer types, generally requiring supplemental braking systems once a trailer surpasses a certain weight.
Supplemental trailer braking systems are generally operated either by an independent control system located in the towing vehicle, or by actuation of the towing vehicle braking system through a coupling of the towing vehicle braking system with the trailer brakes. A coupled braking system is highly desirable because it may function in an essentially automatic mode, freeing the operator of the towing vehicle from the requirement of independently operating two separate braking systems. The resulting ease of operation is particularly important in applications such as recreational trailers, where the operator is generally not a professional driver and may spend a relatively small amount of time driving a vehicle with a heavy trailer. In order for the operator to maintain control of the vehicle-trailer combination, however, precisely coordinated braking is required. If, for example, the trailer brake system should brake harder than the towing vehicle, the trailer may begin to skid and the operator may lose control. Conversely, if the towed vehicle brakes harder than the trailer, the trailer may jackknife, again causing loss of control that may lead to vehicle damage or serious injury. Even if control of the vehicle is not fully lost due to uncoordinated braking problems, the inertia of the heavy trailer may cause the stopping distance of the vehicle/trailer combination to be greatly increased, which also raises a serious safety concern particularly with respect to a relatively inexperienced operator.
A typical supplemental trailer braking system applicable for use in connection with recreational trailers and the like is described in U.S. Pat. No. 4,370,714 to Rettich et al. This system is an “electro-hydraulic” or “electric-over-hydraulic” braking system, that is, a system that uses electrical control signals in order to operate hydraulically-powered brakes. Hydraulic pressure to the trailer brakes is controlled by selectively opening and closing solenoid valves in the trailer hydraulic system. The hydraulics are self-contained within the trailer, so that no interconnections between the towing vehicle and the trailer are required other than electrical connections. The trailer brakes are operated by means of a signal from a pressure transducer connected to the master brake cylinder of the towing vehicle. A pressure transducer in the towed vehicle provides an electrical feedback signal. The control signal and feedback signal are compared to produce an error signal. The brake system coupling is achieved by the effort of the trailer brake control system to minimize this error signal.
U.S. Pat. No. 6,416,138 to Barnett describes another electro-hydraulic, self-contained brake system for trailers. Hydraulic pressure in this system is provided through the operation of an electric motor in a complex arrangement involving a gear and threaded rod. The electric motor operates in response to the signal from a pressure sensor in communication with the master cylinder.
U.S. Pat. No. 5,382,035 to Zbinden describes another electrical control system for hydraulic brakes on a trailer, which includes an electric motor for controlling pressure on a pressure accumulator. The pressure accumulator feeds a fluid control circuit including a pressure-limited valve, which controls a braking jack that actuates the trailer brakes.
Each of these systems suffer from important disadvantages. First, each of these systems are complex, involve a significant number of moving parts, and require the fabrication of customized mechanical components in their construction. Each of these factors increases the cost associated with these brake control systems, and reduces the likely reliability of these systems. What is desired then is a simple, inexpensive control system for a trailer braking system that may be constructed from “off the shelf” mechanical parts, and which is operable to reliably brake a trailer in coordination with and in proportion to the braking applied by the operator of the towing vehicle.
In addition, the systems described above inherently exhibit a brake release signal response time that is slower than the response time to a signal indicating that brakes should be applied. It would be desirable to develop a braking system that can respond very rapidly to both a signal to apply the brakes and a signal to release the brakes.